This has no word export option (unless I missed it) which makes it infeasible for a lot of writers. Like it or not, docx is the format a lot of folks in mainstream publishing expect. Context: My partner is a published author and I've pitched stuff like this at her on multiple occasions as an alternative to Scrivener. In addition to her laughing at my insistence that markdown is a superior way to write, she cites the…
Just to add here, the question is not “is markdown (or tex or binary strings or...) better”, but “can the person who determines whether I get paid open the file?” The fiction publishing world is cutthroat competitive. Trying to use nonstandard tools / “breaking the mold” adds even more friction to success.
But anyway, I do use Markdown and I have submitted to contests etc. that require Word formatted files. For the most part it's an nonissue. I can use Pandoc to export to Word, provide the reference file with the correct format, and boom, I'm done. None of the judges of my works so far have ever had any issues opening my files.
I agree the publishing industry will never accept Markdown. That's ok. I can use the tools I like and still produce what they need to do their part of the job.